A MINIATURE PAN-TILT ACTUATOR - THE SPHERICAL POINTING MOTOR

A pan-tilt mechanism is a computer-controlled actuator designed to poi nt an object such as a camera sensor. For applications in active visio n, a pan-tilt mechanism should be accurate, fast, small, inexpensive a nd have low power requirements. We have designed and constructed a new type of actuator meeting these requirements, which incorporates both pan and tilt into a single, two-degree-of-freedom device. The spherica l pointing motor (SPM)1 consists of three orthogonal motor windings in a permanent magnetic field, configured to move a small camera mounted on a gimbal. It is an absolute positioning device and is run open-loo p. The SPM is capable of panning and tilting a load of 15 grams, for e xample a CCD image sensor, at rotational velocities of several hundred degrees per second with a repeatability of .15-degrees. We have also built a miniature camera consisting of a single CCD sensor chip and mi niature lens assembly that fits on the rotor of this motor. In this pa per, we discuss the theory of the SPM, which includes its basic electr omagnetic principles, and derive the relationship between applied curr ents and resultant motor position. We present an automatic calibration procedure and discuss open- and dosed-loop control strategies. Finall y, we present the physical characteristics and results of our prototyp e.